Electrodeposition



3,057,110 Patented Dec. 4, 1962 3,067,110 ELEQTRGDEPQSITKEN William John Waterman, Heath End, Flacirwell Heath,

and Noel Percy Maliett, Maidenhead, England, assignors to Vandervell Products Limited No Drawing. Filed Mar. 23, 1961, Ser. No. 97,772 8 Claims. (Cl. N L-43) A now well known form of bearing, for use in automotive engines or elsewhere, comprises a layer of an alloy composed substantially of lead and indium, ailixed to and supported by a suitably constituted and fashioned backing. It has hitherto been usual to produce such an alloy layer by first electrodepositing a layer of lead upon the surface of the backing and then electrodepositing a layer of indium upon the surface of the lead layer, after which the two metals are thermally interdiffused. In some cases the lead surface is machined to a predetermined dimension prior to the steps of indium plating and interdiflusion.

The process of manufacturing such bearings would be facilitated considerably by effecting simultaneous deposition of the lead and the indium and a variety of baths and processes have been proposed for effecting such codeposition of lead and indium. Many of these baths include ethylenediaminetetraacetic acid (also known as enta or edta and hereinafter referred to as enta) in at least the stoichiometric quantity required to chelate the total of metals present. In certain cases other chelating agents have been proposed in place of or in combination with enta, but in every case the chelating agent serves to retard the rate of deposition of lead relative to that of indium to a degree which allows of deposition of lead-indium alloys.

An undesirable feature of all such baths is the gradual destruction of the chelating agent by electrolytic oxidation at an insoluble anode. In practice, the cathode efficiency in these baths is always less than that of a soluble anode, so that if soluble anodes only are used, there is an increase in dissolved metal concentration in the solution, whereupon satisfactory control of the alloy being deposited is lost. It has been necessary in general to use an insoluble anode, such as one composed of carbon or graphite, in order to reduce this increase in metal concentration and thereby maintain the metal composition of the bath within a reasonable range.

In the specification of our prior British Patent No. 799,280 we proposed to reduce the descruction of the enta at an insoluble anode by an addition of an easily oxidisable compound such as hydrazine. A bath suitable for the codeposition of lead-indium alloys containing enta and hydrazine may be used for a longer period of time, with insoluble and soluble anodes used alternately, than would be the case in the absence of hydrazine. However, on a commercial scale the gradual destruction of the enta and the increase in undesirable oxidation products acting as cathode inhibitors limit the useful life of the bath.

The present invention provides improved baths and processes for effecting the codeposition of lead and indium suitable for use upon a commercial scale in the production of automotive bearings or for other purposes.

We have discovered that the addition of potassium or sodium cyanide to baths containing both lead and indium compounds exercises a controlling effect on the relative rates of deposition of the two metals such that codeposition of lead-indium alloys is allowed to proceed.

In the baths to be described insoluble anodes are used in conjunction with or alternately with a soluble anode so as to maintain the solution composition within a satisfactory range. There are no undesirable oxidation prodnets to limit the practical usefulness of the baths. The gradual loss of cyanide by oxidation to carbon dioxide and nitrogen, =both harmless to our deposition process, is made good by periodic additions of potassium or sodium cyanide when necessary.

By the practice of the invention layers of lead-indium alloy can be obtained which are so even and smooth surfaced that if deposited upon a bearing backing which has previously been brought to the correct form and dimensions, no subsequent machining operation is necessary upon the bearing surface. However, even in such cases, a finishing operation may be employed and the invention may also be applied to the deposition of leadindium alloy upon backing material which has not yet been brought to the form required to constitute a bearing, or which is intended to serve other purposes.

The following example illustrates the application of the invention to the manufacture of bearings:

Blanks are cut from a strip of suitable backing material, which may consist of a single layer of metal, or of two or more superimposed and bonded layers. The blanks are brought to semi-finished condition by forming, and broaching or boring. The broached or bored surface is smooth and of the correct profile for the electrodeposition step in the process. The semi-finished part is degreased in trichloroethylene vapour and after loading into a suitable jig is plated for the appropriate period without any further preparatory operations. The lead-indium alloy is plated with careful control of the current and time of deposition in order to produce a layer which will bring the semi-finished part up to the required wall thickness.

As an example of the type of electrolyte which we may use for the codeposition step we quote the following bath:

Bath A Indium 16 g./l. (as indium chloride). Lead nitrate 15 g./l. Free potassium cyanide 37 g./l. Free potassium hydroxide 37 g./l. Dextrose 30'g./l. Gelatin 1.0 g./l. pH 12.0. Operating temperature 50 C.

With such a solution at cathode current density of 60 a./ sq. ft. a smooth deposit was obtained containing 26% by weight of indium. An anode consisting of 10% of indium balance lead was used with current efficiency at 30 a./sq. ft.

To maintain the bath composition uniform at suitable anode current densities, it is desirable to employ anodes part of the surface of which is composed of graphite or other insoluble current conducting material, or to alternate the use of soluble and insoluble anodes. Mixed anodes of the pure metals may be employed where convenient in place of alloy anodes.

We have found that additions of a halide salt to our baths is advantageous in allowing the dissolution of a lead-indium alloy anode or of a pure indium anode to proceed at much higher current densities than is possible in the absence of such additions. In this respect potassium fluoride additions are particularly effective, the concentration required in our bath at a given anode current density depending upon the indium content of the anode. In general, the highest concentration of potassum fluoride will be required when pure indium anodes are being used.

We have found that baths may be prepared using compounds of lead and indium other than the nitrate and chloride respectively. Deposits may be obtained for example from solutions containing hydroxides, perchlorates and oxides. Very satisfactory solutions appear to be those based on indium chloride and lead oxide (PbO) with additions of alkali metal nitrates. Nitrate ions will generally improve the homogeneity and smoothness of the deposit. The presence of the nitrate ion always results in a decrease in cathode efficiency. As an example of such a solution we may quote the following bath:

Bath B Indium-.. g./l. (as indium chloride).

Lead oxide 8 g./l. Free potassium cyanide 37.5 g./l. Free potassium hydroxide 37.5 g./l. Dextrose 30.0 g./l. Gelatin 1.0 g./l. Sodium nitrate 5.0 g./l. Potassium fluoride 40 .g./l. pH 10.8. Operating temperature 50 C.

With this bath, operated at a cathode current density of 60 a./ sq. ft. the rate of deposition was 10.8 mgms./ a. min. The deposit was smooth and contained 5.0% by weight of indium. An alloy anode containing 10% by weight of indium dissolved with 100% current efficiency at a./ sq. ft. By way of comparison, a similar bath omitting the sodium nitrate content gave a powdery deposit containing 46% by weight of indium, the rate of deposition being 30.6 mgms./ a. min.

Solutions may also be made using the lead chelate of enta. For example lead oxide, lead chloride or lead sulphate may be added to an alkaline solution containing the minimum of enta required to chelate all the lead being added. The whole may then be added to the plating bath containing the indium. An example of such a bath is as follows:

Bath C Indium 16 g./-l. (as indium chloride). Lead 8 g./l. (as lead chloride). Enta Free potassium cyanide 37.5 g./l. Free potassium hydroxide 37.5 g./l. Dextrose 30.0 g./l. Sodium nitrate 5.0 g./l. pH 12.0. Operating temperature 50 C.

With such a solution at a cathode current density of 60 a./ft. a deposit which contained 2.6% by weight of indium was obtained at a rate of deposition of 18.6 mgms./ a. min. In general, however, we prefer to omit enta from our solutions because of the previously mentioned electrolytic breakdown.

Our improved baths contain a stoichiometric quantity or an excess of alkali metal cyanide. For the alkali metal ion, the potassium ion is preferred because of improved conductivity. The cyanide used in our bath performs the function of complex ion formation. The concentration of simple indium ions is very low, as is evidenced by the fact that indium hydroxide is not precipitated at the high pH values of our baths. The dextrose which is preferably included in the baths also performs the function of complex ion formation in conjunction with the cyanide. We prefer to employ electrolytes containing an excess of cyanide ion concentration over and above that required for complexing of the indium, the cyanide ion content being in the range of 5 to 50 g./l. The dextrose concentration will also be in the range of 5 to 50 g./l.

Our preferred electrolytes contain lead in the concentrations from 1 to g./l. or thereabouts and indium in concentrations of 1 to g./l or thereabouts. The

concentration of alkali metal hydroxide will depend to a large extent on the final pH value of the bath but will generally be in the range of 5 to 50 g./l. or thereabouts. The concentration of potassium fluoride will also be in the range of 5 to 50 g./l. or thereabouts.

The nature of deposits obtained in the absence of a cathodic inhibitor such as gelatin indicates that the presence of this substance is in general desirable. Other colloids such as carpenters glue, agar-agar, gum arabic, have small effects, but gelatin in concentrations in the range 0.1 to 5 g./l. is to be preferred.

The nature of the deposits obtained in the absence of the nitrate ion indicates that the presence of this substance is also desirable. In order to maintain a reasonable deposition rate of lead indium alloy we prefer to limit the concentration of nitrate ion to the range 1 to 6 g./l. Baths will however operate outside this range, but the deposits obtained with less than 1 g./l. of nitrate ion are not suitable for our purpose and, the rate of deposition of lea-d indium alloys fro-m baths containing more than 6 g./l. of nitrate ion is so reduced that the process is rendered uneconomical.

By adjustment of the other operating conditions a wide range of temperature from room temperature up to the boiling point of the electrolyte may be employed. We prefer on considerations of convenient solution conductivity and deposit composition to operate our baths in the range 45 C. to 60 C.

A range of pH values has also been examined and deposits may be obtained between pH 9 and 12.0 or thereabouts. We prefer however to operate in the range between 10.0 and 11.5, as better deposits are obtained in this range.

For the most satisfactory operation of these electrolytes the usual practices of filtration at frequent intervals and the maintenance of a minimum amount of contamination by foreign materials should be enforced. The presence of small amounts of iron in the bath is however not harmful since the deposition of this element at the cathode is retarded in the presence of cyanide.

The plating equipment may be of mild steel, but we prefer to line such equipment with, for example, hard rubber or polyethylene.

We claim:

1. Process for the co-deposition of lead and indium by electrolysis of an aqueous bath having a pH value within the range from 9 to 12 and containing compounds of both metals, at least a stoichiometric quantity of an alkali metal cyanide, from 0.1 to 5 grams per liter of gelatine and at least 1 gram per litre of nitrate ion.

2. Process in accordance with claim 1, in which the bath contains also from 1.5 to 17 grams per liter of fluoride ion.

3. Process in accordance with claim 2, in which the bath also contains from 5 to 50 grams per liter of dextrose.

4. Process in accordance with claim 1 in which the bath contains the following constituents within the respective ranges stated (all in grams per liter) lead 1 to 35, indium l to 50, cyanide ion 5 to 50 and in excess of the amount required for complexing the indium present, alkali metal hydroxide 5 to 50*, nitrate ion 1 to 6, dextrose 5 to 50 and gelatin 0.1 to 5.

5. Process in accordance with claim 4, in which the bath contains additionally from 1.5 to 17 grams per liter of fluoride ion.

6. Process in accordance with claim 1, in which the concentrations of lead and indium in the bath are maintained by using a soluble anode in conjunction with an insoluble anode.

7. Process in accordance with claim 4, in which the bath is at a pH between 10.0 and 11.5.

8. Process for the co-deposition of lead and indium by electrolysis of an aqueous bath having a pH value within the range from 9 to 12 and containing compounds of both metals, at least a stoichiometric quantity of an alkali metal cyanide and at least 1 gram per liter of nitrate ion, said bath being maintained at a temperature of at least 45 C. during the electrolysis.

References Cited in the file of this patent UNITED STATES PATENTS 2,736,692 Eckert Feb. 28, 1956 6 Waterman et al. Apr. 22, 1958 Nobel et a1. June 23, 1959 Hamilton et a1 June 30, 1959 Francisco et a1 Mar. 29, 1960 FOREIGN PATENTS Great Britain May 17, 1950 

1. PROCESS FOR THE CO-DEPOSISTION OF LEAD AND INDIUM BY ELECTROLYSIS OF AN AQUEOUS BATH HAVING A PH VALUE WITHIN THE RANGE FROM 9 TO 12 AND CONTAINING COMPOUNDS OF BOTH METALS, AT LEAST A STOICHIOETRIC QUANTITY OF AN ALKALI METAL CYANIDE, FROM 0.1 TO 5 GRAMS PER LITER OF GELATINE AND AT LEAST 1 GRAM PER LITRE OF NITRATE ION. 